1. Field of the Invention
This invention relates to demodulators, chips and methods for digitally demodulating FSK signals.
2. Background Art
The following references may be referenced herein:    [1] P. R. Troyk, I. E. Brown, W. H. Moore and G. E. Loeb, “Development of BION Technology for Functional Electrical Stimulation: Bidirectional Telemetry,” 23rd IEEE-EMBS Conference Proceedings, Vol. 2, pp. 1317-1320, 2001.    [2] D. G. Galbraith, M. Soma, and R. L. White, “A Wideband Efficient Inductive Transdermal Power and Data Link with Coupling Insensitive Gain,” IEEE TRANS. BIOMED. ENG. Vol. 34, pp. 265-275, April 1987.    [3] C. M. Zierhofer, E. S. Hochmair, “The Class-E Concept for Efficient Wide-band Coupling-insensitive Transdermal Power and Data Transfer,” IEEE 14th EMBS Conference Proc., Vol. 2, pp. 382-383, 1992.    [4] P. R. Troyk and M. Edington, “Inductive Links and Drivers for Remotely-powered Telemetry Systems,” Antennas and Propagation Symposium, Vol. 1, pp. 60-62, 2000.    [5] C. Polk, E. Postow, “Handbook of Biological Effects of Electromagnetic Fields,” Chap. 2, CRC PRESS, 1986.    [6] J. A. Von Arx and K. Najafi, “On-Chip Coils with Integrated Cores for Remote Inductive Powering of Integrated Microsystems,” TRANSDUCERS 97, pp. 999-1002, June 1997.    [7] M. Ghovanloo, K. Beach, K. D. Wise, and K. Najafi, “A BiCMOS Wireless Interface Chip for Micromachined Stimulating Microprobes,” IEEEEMBS Special Topic Conference on Microtechnologies in Medicine and Biology Proceedings, pp. 277-282, Madison-Wis., May 2002.    [8] M. Ghovanloo and K. Najafi, “Fully Integrated Power Supply Design for Wireless Biomedical Implants,” IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology proceedings, pp. 414-419, Madison-Wis., May 2002.    [9] M. Ghovanloo and K. Najafi, “A High Data Transfer Rate Frequency Shift Keying Demodulator Chip for the Wireless Biomedical Implants,” IEEE 45th Midwest Symposium on Circuits and Systems Proc., Tulsa-Okla., August 2002.    [10] M. Ghovanloo and K. Najafi, “A Wideband Frequency-Shift Keying Wireless Link for Inductively Powered Biomedical Implants,” IEEE TRANS. ON CIRCUITS AND SYSTEMS I, Vol. 51, No. 12, pp. 2374-2383, December 2004.    [11] M. Ghovanloo and K. Najafi, “A Modular 32-site Wireless Neural Stimulation Microsystem,” IEEE JOURNAL ON SOLID-STATE CIRCUITS, Vol. 39, No. 12, pp. 2457-2466, December 2004.
The inductive link between two magnetically-coupled coils is now one of the most common methods to wirelessly transfer power and data from the external world to implantable biomedical devices such as pacemakers and cochlear implants [1-3]. However, this is not the only application of data and power transfer via inductive coupling. Radio-frequency identification (RFID), remote sensing, and MEMS are among a few other fields that can benefit highly from this method [4]. Achieving high power transfer efficiency, high data transfer bandwidth, and coupling insensitivity are some of the challenges that one would face in the design of such systems.
Some of the biomedical implants, particularly those which interface with the central nervous system, such as cochlear and visual prostheses, need large amounts of data to simultaneously interface with a large number of neurons through multiple channels. In a simplified visual implant for example, a minimum reasonable resolution of only 32×32 pixels for an image requires 10-bits for addressing, 8-bits for 256 gray levels, and 2-bits for polarity and parity-checking. If one considers the human-eye natural bandwidth of 60 frames/sec, then 1.23 Mbps need to be transferred to this implant just as pure data. Therefore, a high data-rate receiver circuitry that can establish an efficient wireless link between the implant and the external units is highly needed.
In broadband wireless communications such as IEEE 802.11a standard for wireless LAN application, baud rates as high as 54 Mbps have been achieved at the expense of increasing the carrier frequency up to 5.8 GHz, giving a data-rate to carrier-frequency ratio of only 0.93%. In other words, each data bit is carried by 107.4 carrier cycles. On the other hand, the maximum carrier frequency for biomedical implants is limited to a few tens of MHZ due to the coupled coils' self-resonant frequency, more power loss in the power transfer circuitry, and excessive power dissipation in the tissue, which increases as square of the carrier frequency [5]. Therefore, a desirable goal is to transfer each data bit with a minimum number of carrier cycles to maximize the data-rate to carrier-frequency ratio and minimize the amount of power consumption.
So far, amplitude shift keying (ASK) data modulation has been commonly used in biomedical implants because of its fairly simple modulation and demodulation circuitry [1, 3, 4, 6, 7]. This method, however, faces major limitations for high-bandwidth data transfer, because high-bandwidth ASK needs high order filters with sharp cut-off frequencies, whose large capacitors cannot be easily integrated in this low-frequency range of RF applications. A remedy that has been proposed in the so called suspended carrier modulation [1, 3, 4] boosts the modulation index up to 100% to achieve high data rates with low-order integrated filters at the expense of 50% reduction in the transferred power.
FSK data modulation technique has been was utilized for wirelessly operating the University of Michigan micromachined stimulating 3D-microprobes, shown in FIGS. 1a and 1b, which are targeted at a 1024-site wireless stimulating microsystem for visual and auditory prostheses [7,8]. A receiver coil 10, hybrid components 11 and a telemetry interface chip 12 are enclosed within a hermetic package 13. STIM 2/2B probe shanks 14 extend from a substrate 15 which supports electronic circuitry 16 thereon. This implantable microsystem consists of two major parts. First, a series of active (with circuitry) or passive (without circuitry) micromachined stimulating probes that are vertically mounted on a micromachined platform and second, a wireless interface chip that receives data and power through electromagnetic coupling and provides the entire system with regulated power, synchronization pulses and a serial data bit-stream.
FSK Data Transfer
FSK is one of the most common modulation techniques for digital communication, which simply means sending binary data with two frequencies f0 and f1, representing digital “0” and “1” respectively. The resultant modulated signal can be regarded as the sum of two complementary 100% amplitude-modulated signals at different carrier frequencies as shown in FIG. 2a. f(t)=f0(t)sin(2πf0t+φ)+f1(t)sin(2πf1t+φ)  (1)
In the frequency domain, the signal power is centered at two carrier frequencies, f0 and f1, as shown in FIG. 2b. Since f0(t) and f1(t) can have the same amplitude, an excellent characteristic of the FSK modulation for wireless biomedical implants is that the transmitted power is always constant at its maximum level irrespective of f0 and f1 or the data content:
                                                                  f              0                        ⁡                          (              t              )                                                =                                                                        f                1                            ⁡                              (                t                )                                                          =                                                    V                m                            ⇒                                                V                                      r                    ⁢                                                                                  ⁢                    m                    ⁢                                                                                  ⁢                    s                                                  ⁡                                  (                  f                  )                                                      =                                          1                                  2                                            ⁢                              V                m                                                                        (        2        )            
Another difference between the FSK and ASK is that in ASK data transmission the receiver tank circuit frequency response should have a very high quality factor (Q), centered at the carrier frequency to get enough amplitude variation for data detection. However, in FSK data transmission, the pass band should be centered between f0 and f1 with a low Q to pass enough power of both carrier frequencies. This is an advantage for the FSK technique because in the biomedical implant applications, the quality factor of the receiver coil is inherently low particularly when the implant receiver coil is integrated and its high resistivity is unavoidable [6]. The FSK signal is much less susceptible to the coupled coils misalignment and motion artifacts which are two major problems in biomedical implants that adversely affect the amplitude of the received signal.
Synchronization of the receiver with the transmitter is however easier in the ASK systems. Because the receiver internal clock signal can be directly derived by stepping down the constant transmitter carrier frequency [6, 7]. In FSK data transfer, the internal clock with constant frequency can be derived from a combination of the two carrier frequencies (f0 and f1) based on the data transfer protocol or synchronization patterns.
One wants to maximize the data-rate to carrier frequency ratio. Therefore, a particular protocol was devised for the FSK data transfer with the data-rate as high as f1 with f0 twice as f1. In this protocol, the digital bit “1” is transmitted by a single cycle of the carrier f1 and the digital bit “0” is transmitted by two cycles of the carrier f0 as shown in FIG. 2a. The transmitter frequency switches at a small fraction of a cycle and only at zero crossings. This leads to a consistent data transfer rate of f1 Bits/sec. As a result, if one considers the average carrier frequency to be (f0+f1)/2, then the data-rate to carrier frequency ratio can be as high as 67%. It is also useful to notice that any odd number of consecutive f0 cycles in this protocol is an indication of data transfer error.
The following U.S. patents are related to the invention: U.S. Pat. Nos. 5,684,837; 4,616,187; 3,611,298; 3,623,075; 4,021,744; 3,979,685; 3,846,708; 3,908,169; 5,533,061; 4,115,738; 3,660,771; 5,550,505; 4,551,846; 3,600,680; 5,399,333, 4,488,120; 5,649,296; 4,485,347; 4,368,439; 4,825,452; 6,144,253; 4,103,244; 4,987,374; 5,748,036; 5,155,446; 4,568,882; 4,752,742; 5,245,632; 4,486,715; 4,533,874; 4,529,941; 6,359,942; 5,724,001; 6,038,268; 3,636,454; 6,501,807; 3,539,828; 4,010,323; 5,953,386; 4,773,085; 3,512,087; 3,501,704; 3,947,769; 5,053,717, 3,614,639; 3,949,313; 4,485,448; 5,436,590; 5,394,109; 4,716,376; 5,309,113; 3,427,614; 5,583,180; 4,363,002; 4,513,427; 6,307,413; 5,317,309; 6,122,329; 3,740,669, 4,451,792, 5,781,064; 5,329,258; 3,773,975; 3,991,389; 5,621,755; 6,366,135; 5,105,466; 3,372,234; and 4,543,953.
References [9] and [10] disclose a high-rate frequency shift keying (FSK) data transfer protocol and demodulator circuit for wirelessly operating biomedical implants in need of data transfer rates above 1 Mbit/Sec. The demodulator circuit receives the serial data bit stream from an FSK carrier signal in 2-20 MHZ range, which is used to power the implant through inductive coupling.
The data detection technique used for the FSK demodulation is based on measuring the period of each received carrier cycle. If the period is higher than a certain value, a digital “1” bit is detected and otherwise a digital “0” is received. Time measurement is provided by charging a capacitor with a constant current source and monitoring its voltage. Charging and discharging of this capacitor is synchronized with the FSK carrier signal. If the capacitor voltage is higher than a certain value, a digital “1” bit is detected and otherwise a digital “0” is received. This comparison can be done in two ways:                1. Fully differential FSK demodulator (FDFSK): Charging two unequal capacitors with different currents and compare their voltages with a hysteresis comparator. This is like comparing two capacitive timers with different time constants.        2. Referenced Differential FSK Demodulator (RDFSK): Generating a reference voltage and comparing it with a charging capacitor voltage.        
Such a demodulator circuit is usually a part the analog portion of a mixed signal chip and, in some cases, it is the only analog block on the chip. Analog blocks usually occupy more area than their digital counterparts. Specifically, the common FSK demodulation techniques need some kind of analog filtering down the signal path, which consume even more chip area due to the above-noted low-end RF application. Analog circuits are more susceptible to process and temperature variations and their design becomes more challenging with the trend towards smaller feature size and lower power supply voltage. Therefore, a fully digital demodulator can save a lot of chip area, make the system more robust, and ease many of the above problems.